Hydraulic fracturing is used to stimulate the production of hydrocarbons such as oil or natural gas from subterranean formations. In this process, a fracturing fluid is injected through a wellbore and propelled against the formation strata by high pressure sufficient to cause the strata to crack and fracture.
The fracturing fluid also carries what is referred to as a proppant to the fracture site. These proppant particles remain in the fracture thereby “propping” the fracture open when the well is in production. The proppant material is commonly sand, sintered bauxite, glass balls, polystyrene beads, and so forth.
Traditional fracturing fluids were polymer based fluids. Naturally occurring polysaccharides such as guar and derivatized guar, upon dissolution in water, form hydrated polymers which can be crosslinked with organometallic compounds such as borates, zirconates or titanates to form a crosslinked gel structure which increases the viscosity of the fluid. However, these crosslinked polymers can ultimately form a rubbery semi-solid material that is difficult to recover from the fracture causing reduction in the conductivity of the fracture. Peroxides and other gel breakers are therefore often utilized to break the gelled polymeric structure into smaller molecules and facilitate clean-up of the fracture site. Production of oil or gas is often dependent on the ability to clean the fracture site of such materials.
It is a widely known concept to use certain chemical agents to increase the viscosity or induce gel formation in aqueous or hydrocarbon based fluids. These gelled or thickened fluids may be referred to as viscoelastic fluids. The term viscoelastic is commonly understood to refer to a fluid that has the capability to partially return to its original non-disturbed form upon release of applied stress. It is believed that under the right conditions, i.e. concentration, counter ion, pH and so forth, that certain surfactants from rod-like or cylindrical micelles which can entangle to form these gelatinous, viscoelastic fluids in aqueous solutions.
These thickened fluids are then capable of suspending the proppant particles and carrying them to the fracture site. These gelled fluids also diminish loss of fluid into the fracture face, thereby improving the efficiency in the fracturing process and reducing the overall cost.
Viscoelastic fluids can be formed with a combination of ionic and/or non-ionic surfactants and by the incorporation of additives such as organic and/or inorganic salts. U.S. Pat. No. 5,551,516 issued Sep. 3, 1996 to Norman et al. describe surfactant based viscoelastic aqueous fluid systems useful in fracturing subterranean formations penetrated by a wellbore, and state that the fracturing fluids are useful in high temperature, high permeability formations. Norman et al. state that the preferred thickening agents are quaternary ammonium halide salts derived from certain waxes, fats and oils. The thickening agent is used iii conjunction with an inorganic water soluble salt, an organic stabilizing additive selected from the group of organic salts such as sodium salicylate. The fluids are said to be stable to a temperature of 225° F.
Various problems have been encountered with these gelled fluids in oil field applications including the lack of thermal stability of the fracturing fluids caused by the degradation of the additives or the instability of the gel upon exposure to high temperatures and/or high shear conditions. This can result in changes in the rheological properties of the gel which can ultimately affect the ability of the fluid to suspend proppant material. If proppant material is prematurely lost from the fracturing fluid it can have a detrimental effect on the fracturing treatment. Furthermore, gel instability can result in higher loss of fluid into the formation thereby diminishing the amount of fracturing that occurs. High temperatures in the formation can further impact gel stability.
For high brine solutions, i.e. those having salt concentrations of about 30 wt-% or more, polymeric materials have been used as thickeners but tend to precipitate from these high salt concentration solutions which can result in plugging and formation damage. Furthermore, the high salt concentration makes it difficult to dissolve these polymeric viscosifiers or thickeners in the solutions. Such high brine solutions are found in completion fluids used in drilling operations.